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Entry  Wed May 13 16:57:28 2009, Dmass, Laser, PMC, Relocking the PMC 
    Reply  Thu May 14 20:57:22 2009, rana, Laser, PMC, Relocking the PMC 
       Reply  Fri May 15 04:23:19 2009, Dmass, Laser, PMC, Relocking the PMC Pmclock.png
Message ID: 128     Entry time: Fri May 15 04:23:19 2009     In reply to: 127
Author: Dmass 
Type: Laser 
Category: PMC 
Subject: Relocking the PMC 
Let there be lock. Picture included below.

  • I have now added a 95% Beamsplitter right after the PBS to dump most of the power into another big beam dump from the 40m
  • The ZAD-6 is now geetting exactly 7dBm of 35MHz from the Marconi
  • I put a DC Block on the PD Output before the mixer.
  • I terminated the input into the DAC with a 50 Ohm terminator at the DAC off a T, after the LP Filter
  • I stopped using the SR 560
  • The digital world has a single pole at 1 Hz toggled. The ouput goes to the PZT drive box.
I Also:
  • Removed an extra mirror from in front of the refl PD which was giving me real low optical gain. I have now all of refl (25ish mW) on the PD.
  • Realigned "by eye" (dithered a drive signal about the 00 mode and did a really low BW servoing.
  • Swapped in and out a pi/2 phase shift onto my drive every time I changed anything, since I have no idea if the phase is right.
  • Changed the sign of my feedback any time it was obvious it was doing something ridiculous (I think I got a sign flip through the SR560 one time, and this may have been the only instance of having to insert a negative)
  • Changed the offset slowly by hand when it would start to hang out on the 00 fringe to try to force it to find the peak.

W.A.G.s and musings
  • The mode overlap between the cavity and input beam is awful (crude alignment and no tune up of modematching means that I might be very mismatched in all 4 degrees of freedom). Maybe that means no good change in my signal on refl PD. Test this by looking at Ptrans with another photodiode.
  • There is blue sharpie on the ZAD-6 mixer indicating that it wants +1 dBm of RF. When I plug in this signal (after the DC block) I don't see anything but noise in an HP4395A (I would expect some 35 MHz signal from the pre demod signal...I think) Do I need an amplifier or a different mixer?
  • There is something stupid that I am not doing. The sign of my feedback was wrong after I had fixed everything else.
  • There are two beams from the PSL that are roughly parallel (see a couple elogs back about scans). If I am hitting my PMC with both of these, there might be weird coupling going on between the misaligned one and the sidebands that puts lots of informationless 35 MHz signal on my photodiode. If this is the case we might need to open up the PSL and realign things until we aren't tempted to use more than one eigenbasis when talking about the modal content.
  • There is something clever that I am not doing.

  • Instead of using the polarizer all the way, set the polarizer to transmit a few W and then use a fixed BS to attenuate down to 10-30 mW before doing the PMC alignment.
  • The ZAD-6 is a level 7 mixer so you have to give it 7 dBm. So there should be no attenuator after the splitter for the LO.
  • Have to AC couple (or DC block) the PDA signal before going into the mixer. It wants AC only.
  • The mixer output should go into the 1.9 MHz low pass and then get terminated with 50 Ohms (like with a T). Then the other output of the T can go to the DAQ or into the SR560 for some gain but you shouldn't need very much gain. Certainly the SR560 should have no filter clicked on.
  • If you use the SR560, make sure the output is zero with no light on the PD. There's a trim pot on the front.
  • In the digital world there should be a single pole around 1 Hz. Make sure the output actually goes to the PZT driver.
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